With rapid development of various types of electronic devices, there is a tendency that these devices and products are smaller in size and larger in capacitance. The component parts for these devices are also increasingly produced in compact and lightweight design. The means for mounting the electronic components are also changing to surface mounting technology (SMT). Small components such as capacitor and resistor are called “chip components”.
Multilayer Ceramic Capacitor (MLCC) is a widely used typical chip component. It is generally fabricated by forming alternating layers of an internal electrode forming paste and a dielectric layer-forming paste. Such layers are typically formed by sheeting, printing or similar techniques, followed by concurrent firing.
The Electronic Industry Association (EIA) prescribes a standard for temperature coefficient of capacitance (TCC) for a type of MLCC known as the X5R/X7R capacitors. The X5R characteristic requires that with reference to the capacitance at 25° C., the change of the capacitance should be within ±15% over the temperature range from −55° C. to 85° C., and the dielectric loss is no more than 2.5%. The X7R characteristic requires that with reference to the capacitance at 25° C., the change of the capacitance be within ±15% over the temperature range from −55° C. to 125° C., and the dielectric loss is no more than 2.5%.
When MLCC based on BaTiO3 were sintered in air at high temperatures, it is required to use noble metals (e.g., palladium, platinum, etc.) as internal electrodes, which do not melt and do not oxidize even when being fired in an atmosphere with a high partial pressure of oxygen. However, use of such noble metals becomes a barrier to cut down the production cost of multilayer ceramic capacitors. For example, the cost of internal electrodes occupies about 30 to 70% of the production cost of multilayer ceramic capacitors. For the reasons mentioned above, it is preferred to use a base metal such as Ni, Cu as the material of internal electrodes. However, if such base metals are used as a material for internal electrodes and fired in the conventional firing conditions of the dielectric ceramic materials, they would be oxidized easily and lose functions as the internal electrodes. Thus, in order to use such a base metal as a material for internal electrodes of multilayer ceramic capacitors, it is required to dope some elements such as Mn or Mg to avoid semiconductorization of BaTiO3 based dielectric material, and required to be fired in a neutral or reducing atmosphere with a low partial pressure of oxygen, so that to ensure sufficient insulation resistance and good dielectric properties.
To meet the requirements above, it has been proposed some non-reducible dielectric ceramic materials. For example, JP-A-63-103861 discloses a composition of BaTiO3—MnO—MgO-Rare earth element system. However, this dielectric ceramic composition is of no practical use since its insulation resistance and temperature coefficient of capacitor are affected by the grain size of the main component BaTiO3, thus making it difficult to control the composition to obtain stable dielectric properties.
In US 20040229746A1, a composition of BaTiO3—Mn3O4—Y2O3—Ho2O3—CaCO3—SiO2—B2O3—Al2O3—MgO—CaO is disclosed. Although it can be sintered at 1200° C. to 1300° C., the grain size is larger than 500 nm, which is not in favor of reducing the thickness of the dielectric layers.
In DE-19918091A1, a composition of BaTiO3—MgO—MnO—V2O5—Al2O3—Ho2O3—BaCO3—SrO—CaO—CoO—ZrO2 is disclosed. It satisfies the X7R characteristics: the dielectric constant can be adjusted at 2000-4000. However, the sintering temperature for this system is too high, over 1300° C. and the TCC at −55° C. or 125° C. is near −15%. Thus, the composition is not suitable for industrial production.
The development of electronic equipments towards miniaturization and high performance requires smaller multilayer ceramic capacitors (MLCCs) for industry application. The thickness of dielectric layer becomes thinner and thinner, from 10 μm to 5 μm, 2 μm, 1 μm, and even below 1 μm. In order to ensure the reliability of MLCCs, the corresponding grain size should be reduced from 1000 nm to 500 nm, 200 nm, 100 nm, and even smaller. Meanwhile, the grain size should be very uniform. However, the reducing of the grain size generally induces the decreasing of the dielectric constant. In U.S. 62/709,906B1, when the grain size of the ceramic reduces to 100 nm to 200 nm, the dielectric constant is 1600 to 1800, lower than 2000.
Therefore, the problem that this invention intends to solve is to control the composition, microstructure and sintering process of the dielectric ceramic material to obtain a dielectric material suitable to be used for an ultrafine grained and temperature-stable multilayer ceramic capacitor.